Human tremors are some of the most common movement disorders. Human tremors can be associated with several factors, such as ageing and certain neurological disorders, including Parkinson's Disease (PD). PD is a degenerative disorder of the central nervous system resulting from the death of dopamine-generating cells in the substantia nigra leading to movement-related and, later, cognition-related symptoms.
Transcranial Magnetic Stimulation (TMS) is a safe and non-invasive method that affects the cerebral cortex but not deep structures. TMS uses electromagnetic induction by means of a rapidly changing magnetic field to induce weak electric currents that provoke activity in specific or diffuse parts of the brain. TMS causes depolarization or hyperpolarization in the neurons of the brain and can be used to investigate causality in the brain-behavior relationship.
Single/paired-pulse TMS depolarizes and discharges the action potential in the region that is being stimulated. When applied to the motor cortex, single/paired-pulse TMS can provoke motor evoked potentials. A variant of TMS is repetitive TMS (rTMS), which produces more durable effects after the initial stimulation. rTMS can increase or decrease the excitability of the corticospinal tract depending on the intensity of stimulation, coil orientation, and frequency.
An excitatory effect of TMS in one area may (through inhibitory interconnections) induce inhibition of a different area. TMS techniques can be used to diagnose movement disorders and as a therapeutic tool in PD. For example, studies have indicated increased inhibition in patients with PD following TMS suggesting that cortical excitability is reduced.
Previous studies in PD patients have shown that rTMS can reduce impairment in PD. For example, at rTMS frequencies of 5 Hz and higher, enhanced motor cortex excitability was observed, whereas lower frequencies of 1 Hz and lower transiently depressed cortical excitability.